Mesomorphic behavior of new benzothiazole liquid crystals having Schiff base linker and terminal methyl group

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Mesomorphic

behavior

of

new

benzothiazole

liquid

crystals

having

Schiff

base

linker

and

terminal

methyl

group

Sie

Tiong

Ha

a,

*

,

Kok

Leei

Foo

a

,

Hong

Cheu

Lin

b

,

Masato

M. Ito

c

,

Kazuma

Abe

c

,

Kenji

Kunbo

c

,

S. Sreehari

Sastry

d

a

DepartmentofChemicalScience,FacultyofScience,CentreforBiodiversityResearch,UniversitiTunkuAbdulRahman, JlnUniversiti,BandarBarat,31900Kampar,Perak,Malaysia

b

DepartmentofMaterialScience&Engineering,NationalChiaoTungUniversity,Hsinchu300,Taiwan,China

c

FacultyofEngineering,SokaUniversity,1-236Tangi-cho,Hachioji,Tokyo192-8577,Japan

d

DepartmentofPhysics,AcharyaNagarjunaUniversity,NagarjunaNagar522510,India Received15February2012

Availableonline9June2012

Abstract

Ahomologousseriesofheterocycles,6-methyl-2-(4-alkoxybenzylidenamino)benzothiazoles,weresynthesizedand character-izedusing FT-IR,1_H_and 13_C _NMR_and_mass _{spectrometric} _analysis._{Enantiotropic}_nematic_phase_was_observed_for_shorter

members.SmecticAphaseonlyemerged fromoctyloxyderivativeonwards.Theterminalmethylgroupatthebenzothiazole fragmentandtheSchiffbaselinkageinfluencedthemesomorphicbehaviorofthepresentseries.

#₂₀₁₂_Sie_Tiong_Ha._Published_by_Elsevier_B.V._on_behalf_of_Chinese_Chemical_Society._All_rights_reserved.

Keywords: Schiffbases;Benzothiazole;Liquidcrystal;SmecticA;Nematic

Overthe pastthirtyyears,liquidcrystalshave becomethequintessentialmolecularelectronic materialsof our

presentdayera.Theirapplicationsencompassresearchactivitiesinthefieldofsurfactantanddetergents,membrane

for airseparation, highstrength polymers, heat resistant materials,photonic, thin films,semiconductors etc. [1].

Heterocyclesareofgreatimportanceascoreunitsinthermotropicliquidcrystalsduetotheirabilitytoimpartlateral

and/or longitudinal dipoles combined with changes in the molecular shape [2]. A great deal of mesomorphic

compoundscontainingheterocyclicunitshavebeensynthesized,andinterestinsuchstructuresisconstantlygrowing

[2,3].However,onlyscantinformationontheinclusionofbenzothiazolecoreintoliquidcrystalsystemisavailable

[4–6].Funahashi andHannahavereportedthefastholetransportpropertyofthephotoconductivecalamitic liquid

crystal, 2-(4-heptyloxyphenyl)-6-dodecylthiobenzothiazole [7].A benzothiazole ring containing the electron-rich

sulphuratomcancontributetoalowionizationpotentialandalsoinduceasmecticphase.Theflatmolecularshape

withlittleinterannulartwistinginfusedheterocyclicringsmayalsofacilitateahighdegreeofoverlappingmolecular

wave function for an efficienthopping mechanism of charge transport [8].Benzothiazole unit was alsofound in

fluorescentcompounds,whichisusefulinapplicationsasaresultofhighfluorescencequantumyieldsinthepresence

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ChineseChemicalLetters23(2012)761–764

*Correspondingauthor.

E-mailaddresses:[email protected],[email protected](S.T.Ha).

1001-8417/$–seefrontmatter#₂₀₁₂_Sie_Tiong_Ha._Published_by_Elsevier_B.V._on_behalf_of_Chinese_Chemical_Society._All_rights_reserved.

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oftherigidcorestructure[9].Recently,benzothiazolederivativeshavebeencontinuouslyinvestigatedforapplication

inthin-film, organicfield-effecttransistors[10].

InordertofurtherexplorebenzothiazoleasamesogeniccoreinSchiffbaseliquidcrystals,here,wereportedanew

homologous series of 6-methyl-2-(4-alkoxybenzylidenamino)benzothiazoles. The synthetic route for the

benzothiazole derivatives is illustrated in Scheme 1. 2-Amino-6-methylbenzothiazole and 2,

4-dihydroxybenz-aldehydewerecoupledbyrefluxinethanolforthreehours,followingwhichtheSchiffbaseintermediatewassubjected

toWilliamsonetherificationwiththeappropriatebromoalkanesinthepresenceofpotassiumcarbonateanhydrous.

Thecrudeproductswerepurifieduponrepeatedrecrystallizationusingethanolandtheirstructureswereelucidatedvia

elementalanalysis,FT-IR,NMRandEI-MSspectroscopictechniques [11].

Theliquidcrystallinetexturesoftheproductswereobservedunderapolarizingopticalmicroscopeequippedwitha

hotstageandtemperatureregulator.Phaseidentificationwas madebycomparing theobservedtextures withthose

reportedintheliterature[12,13].Transitiontemperaturesandassociatedenthalpychangesweredeterminedusinga

differentialscanning calorimeter.The resultsaresummarizedinTable1.

Underthepolarizingmicroscope,shortestmemberoftheseries,6MeBTHexhibitednematicphase.Themesophase

wasidentifiedbythenematicdropletstexture.Brownianflashes,afeatureofthenematicphasewerealsoobserved

beforerecrystallization[14].Byreferringtothechemicalconstitutionofthepresentseries,thenematicphasewhich

wasobservedcouldhaveresultedfromtheterminalmethylgroupatthesixthpositionwhichdisruptedthelamellar

arrangementofthesmecticphase.Thesimilarphenomenonwasalsoreportedforahomologousseriesofazoliquid

crystals witha benzothiazole core [5].Furthermore, the Schiff base(CH N) linker whichconferred astep like

structureresultedinthe thickeningeffectwhichinturnenhancedthe nematicphasestability[15].

Aninterestingphenomenonwasobservedfor8MeBTHwhereanadditionalphasewithfan-shapedtextureassigned

assmecticA(SmA)phase(Fig.1b)wasobservedaftertheformationofthenematicdroplets(Fig.1a)onthecooling

run.SmAphaseemergedasmonotropicphase(metastable)in8MeBTH.FromC10derivativeonwards,SmAphase

S.T.Haetal./ChineseChemicalLetters23(2012)761–764 762

+

O OH H C H₃ NH₂ N S H3C N N S C OH C H₃ N N S C OC_nH_2n+1 K₂CO₃ CH₃COCH₃ C_nH_2n+1Br C₂H₅OH CH₃COOH nMeBTH n = 6,8,10,12,14 Scheme1. Syntheticrouteforthetargetcompounds.

Table1

PhasetransitionandtransitionenthalpyfornMeBTHuponheatingandcoolingcycle.

Compound Phasetransition,8C(correspondingenthalpychanges,kJmol 1) Heating Cooling 6MeBTH Cr105.5(39.83)N119.5(1.03)I Cr83.3(35.60)N119.4(1.47)I 8MeBTH Cr108.2(49.10)N118.3(1.33)I Cr70.4(33.94)SmA96.2(1.43)N118.2(1.84)I 10MeBTH Cr99.0(49.84)SmA110.7(3.41)N118.4(1.79)I Cr61.1(37.42)SmA110.8(3.13)N118.1(1.94)I 12MeBTH Cr81.6(44.98)SmA116.1(8.25)I Cr68.9(41.13)SmA116.7(8.90)I 14MeBTH Cr83.2(48.79)SmA113.6(8.6)I Cr69.7(47.86)SmA115.1(8.97)I

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existsasenantiotropicphase(stable)andpersiststoC14derivative.Highermembersoftheseries(C12andC14)

displayedonly smecticphase.This suggestedthat withincreasingthelength oftheterminal chain,thenematic

properties decreased andledto theemergenceofthesmectic phase.Thisis duetoattraction betweenthe long

alkyloxy chainsleading totheir intertwining,whichfacilitates thelamellar packingand isvital forthesmectic

phase[14].It canthereforebe proposedthatinorder togenerate theenantiotropic(stable) smecticphaseinthe

analogoussubstitutedC7H5SNCH NC6H5compounds,thenumberofcarbonsinthealkylchainmustbeatleast

10 (n10) [16].

Aplotofthetransitiontemperatureagainstthenumberofcarbonsinthealkyloxychainduringtheheatingscanis

depictedinFig.2.Themeltingtemperaturewasconsiderablyreducedbytheincreaseinthelengthofthechainowing

to the increase in its flexibility [15]. As for clearing temperature, it descended as the number of carbon atoms

increased, resultingfrom thedilutionofthe coreinduced bytheincrease inthe lengthof theterminalchain[17].

Furthermore,the nematicphaserange isreducedandthe SmAphaserange isincreasedas thealkylchainlength

ascended.TheincreasingvanderWaalsforcestendtostabilizetheSmAphasebyfavoringthelamellarpacking,onthe

otherhand,suppressedthenematicphaserange.

Inconclusion,allthesynthesizedcompoundsexhibitedmesomorphicpropertieswherebynematicphaseexistsfor

theshortermembersandSmAphaseemergedfromtheC8derivativesonwards.Thepresenceoftheorderedsmectic

structureinthetitlecompoundsbecomespotentialinterestinelectricalstudiesfordeviceapplication.

Acknowledgment

The author (S.T. Ha) would liketothank UniversitiTunku Abdul Rahman andMinistry of Higher Education

(MOHE)for thefinancialsupportsvia LRGS(No.LR003-2011A)andresearchfacilities.

Fig.1. Opticalphotomicrographsof8MeBTHuponcooling.Formationofnematicdroplets(a)cooledfromisotropicanduponfurthercooling, SmecticAphasewithfocalconictexture(b)wasobserved.

70 80 90 100 110 120 130 14 12 10 8 6

Number of carbon atoms (n) in alkyloxy chain

Transition Temperature (deg Celcius)

Cr-SmA/N SmA-N SmA/N-I Cr N I SmA

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